Known in the art is an internal combustion engine arranging an NOx selective reducing catalyst able to reduce NOx in the presence of ammonia in an engine exhaust passage, arranging a reducing agent feed valve for feeding a urea aqueous solution into the engine exhaust passage upstream of the NOx selective reducing catalyst, and reducing the NOx contained in the exhaust gas by the urea aqueous solution fed from the reducing agent feed valve in the NOx selective reducing catalyst (see for example International Patent Publication WO99/67511). In this internal combustion engine, the ammonia formed from the urea aqueous solution fed at the NOx selective reducing catalyst is used to reduce the NOx contained in the exhaust gas.
However, it is known that in this NOx selective reducing catalyst, the ammonia formed from the urea aqueous solution is adsorbed once at the NOx selective reducing catalyst and the adsorbed ammonia reacts with the NOx whereby a reduction action of NOx is carried out. On the other hand, exhaust gas contains hydrocarbons, that is, HC. The HCs also try to be adsorbed at the NOx selective reducing catalyst.
That is, if feeding a urea aqueous solution upstream of the NOx selective reducing catalyst, the ammonia formed from the urea aqueous solution and the HC contained in the exhaust gas compete with each other to be adsorbed at the NOx selective reducing catalyst. In this case, if the amount of HC contained in the exhaust gas becomes large, the amount of adsorption of HC at the NOx selective reducing catalyst increases and as a result the amount of adsorption of ammonia at the NOx selective reducing catalyst is reduced, so the reduction action of the NOx at the NOx selective reducing catalyst ends up being weakened. As a result, there is the problem that a high NOx purification rate by the NOx selective reducing catalyst cannot be obtained.